Electrochemical machining (ECM) is a commonly used method of machining electrically conductive workpieces with one or more electrically conductive tooling electrodes. During machining, a tooling electrode is located near the workpiece, such that a gap is defined therebetween, that is the machining gap. The gap is filled with a pressurized, flowing, aqueous electrolyte, such as a sodium nitrate aqueous solution. For a given tooling electrode geometry, dimensional accuracy of the workpiece is primarily determined by the distribution of the gapping. The gap size should be maintained at a proper range. Too small a gap, such as less than 100 micrometers in a standard ECM operation, could lead to short-circuiting between the tooling electrode and the workpiece. Too large a gap could lead to excessive gap variation, as well as reduction in the machining rate. Monitoring and controlling the gap size between the tooling electrode and the workpiece is important for ECM.
Lack of suitable means for detecting gap size may hinder ECM accuracy control. Without such means, many rounds of costly trial-and-error experiments must be run to obtain the gap size changes that occur during the machining process. Gap size can change significantly during the machining process, partly because conductivity of the electrolyte may change in the gap due to heating or gas bubble generation on the tooling electrode surface. Variation and inaccuracy in tooling electrode feed rate and tooling electrode positioning can also contribute to changes in gap size and workpiece thickness. In-process gap detection is important for improving ECM process control.